Machines for the production of compression and/or tension springs may be either of a purely mechanical type or a type involving numerical control, and be provided with one or two noses for winding or forming the turns of the spring, so as to impart to the spring precise dimensions and the required initial stress. The procedure sometimes involves, in terms of the equipment of the machine, a suitable initial fixed rotary movement of one or both of said winding noses, for the purposes of achieving for example the desired characteristics in regard to planarity of the end turns and of closing them up. In particular, in machines provided with two winding noses, the upper winding nose is mostly rotated in the case of right-hand coil springs while the lower nose is mostly rotated for left-hand coil springs.
Since normally the wire slides in the longitudinal direction within a short groove or channel portion provided on the end of each winding nose, the rotary movement of the winding nose about its own axis requires the wire to slide against two opposite edges of the sides of the groove, causing the wire to experience an orientation effect such as to permit it to achieve the desired preload value in the finished product.
The preload value or initial tension can vary greatly for compression springs or tension springs, where the term xe2x80x98initial tensionxe2x80x99 used in this art generally denotes the fact of imparting to the wire a tendency to hold the turns of the spring in a condition in which they are closed up together.
For normal compression springs which do not require grinding operations it is sufficient to impart only a modest initial stress in order to achieve an acceptable arrangement of the end or terminal turns. In the case of springs which require a grinding operation to give flat ends, the initial tension has to be higher insofar as the grinding operation causes a rise in temperature of the wire with relaxation of the stresses internal to the material and thus possibly causing undesired opening of the ends. Finally for tension springs which are already characterised by a constant preload along the entire extent of their body, a high or very high initial tension is generally required.
In any case, in terms of the equipment of the machine, the rotary movement of at least one of the winding noses for achieving a correct value in respect of the initial stress, if the at least one winding nose has remained fixed for the entire production cycle, involves various disadvantages such as for example:
a) in cylindrical compression and tension springs, the first turn has a shape which is curved in the axial direction, at a position corresponding to the initial end of the spring, and
b) in the case of tapered springs with a spring body which is either straight or concave or convex, which, besides fault a), always involve a gradual decrease in the initial stress as the diameter of the spring increases, the fixed correction which applies in respect of the first turns of the spring is no longer sufficient for the last turns of larger diameter, giving rise to possible opening effects at the final end of the spring. Obviously the more the initial stress imparted to the turns of the spring is accentuated, the greater will be the amount of undesirable deformation phenomena at the initial end of the spring; vice-versa, an excessive reduction in the initial stress for nullifying such deformation at the initial turn involves the certainty of the final end of the spring opening.
c) In general, all compression springs with medium or high pitch values bear scrape marks on the wire which are due to the high contact pressure which is generated at the edges of the passage of the winding nose, as a result of the high variation in the helical angle as between the end turns and the spring body. Both (i) those scrape marks on the wire and (ii) the consequential wear of the edges of the passage of the winding noses can be avoided only by virtue of variable orientation of the winding nose in dependence also on the pitch, something which is impossible at the present time with just the stratagem of providing a fixed initial orientation for the winding nose.
The aim of the invention is that of controlling the stress of the wire throughout the entire process of forming the spring by continuously varying it in such a way as to obviate the above-mentioned faults.
In accordance with the invention the method provides for continuous angular correction of the winding nose in machines which are provided with a single nose or of at least one of the winding noses in machines which are provided with several winding noses, in the course of the production cycle of each individual spring, such correction being effected in accordance with laws dictated by calculation and/or practical tests and being repetitive with a cyclic return to an initial position. Repetition of the cycle can be easily linked for example in accordance with the length of the wire which is supplied for producing an element or by the position of the cutting blade which shears the wire at the end of the spring cycle.
A further advantage which derives from the fact of controlling the rotary movement of the winding nose is that of also making it possible to implement compression springs with modest pitch values without the use of the tool (divider) for achieving the desired pitch; it is in fact possible in those cases to give the pitch to the spring by the simple rotary movement of the winding nose, avoiding equipping the machine with the divider tools.
The mechanism for controlling the rotary movement of the winding nose in accordance with the method of the invention can for example be one of three types:
1) the shaft bearing the winding nose for the turns of the spring may be coupled to a suitable mechanical system involving a cam and a lever;
2) the shaft bearing the nose may be rotated by means of a lever moved by a piston which is pneumatic or oleodynamic, under the control of actuators; and
3) the shaft of the nose can be moved directly by an electric motor which is possibly provided with a reduction unit and controlled electronically in accordance with a preset program.